Known sintered bodies which have been used in many applications as thermally stable ceramics include oxides such as Al.sub.2 O.sub.3, BeO, MgO, ZrO.sub.2 and SiO.sub.2, carbides such as SiC, TiC, WC and B.sub.4 C, nitrides such as Si.sub.3 N.sub.4, BN and AlN, borides such as TiB.sub.2 and ZrB.sub.2, silicides such as WSi.sub.2 and CrSi.sub.2, and ternary or higher compounds of these. These ceramic sintered bodies have been produced by shaping powders of these and sintering the shaped powders at extremely high temperatures.
Investigations have been extensively undertaken in recent years with a view to producing less porous high-density sinters at relatively low pressures or relatively low sintering temperatures by using certain additives. If suitable additives are used, the self-sinterability of ceramics can be increased and simultaneously, the abnormal growth of the particles of sintered bodies can be inhibited to prevent the remaining of pores among the particles. Furthermore, the grain boundary can be filed with the additives in high density. Accordingly, high-density sintered bodies can be obtained with economical advantage.
Additives of this kind which have been used heretofore include, for example, MgO and NiO for Al.sub.2 O.sub.3, CaO and TiO.sub.2 for ZrO.sub.2, Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 for Si.sub.3 N.sub.4, B, Si and C for SiC, Ni and WC for TiC, and ZrO.sub.2 and CrB.sub.2 for ZrB.sub.2. Thus, any of such additives are oxides. Although less frequently, metallic elements are also used as the additives, and moreover, for example, for a given carbide, another carbide is added and for a given boride, another boride is added. The reason for selecting these additives is that an interaction between the substrate ceramic and such an additive takes place so as to facilitate the sintering of the ceramic having poor self-sinterability, or that such an additive becomes plastic or liquid at high temperatures so that the sintering of the ceramic easily proceeds.
These conventional additives, however, have various defects as will be described below. In the production of a high-density sintered body by a solid-phase reaction between the additive and a ceramic substrate, second and third phases occur as a result of reaction between the additive and the ceramic and are present mainly in the crystal grain boundary. When the temperature becomes high, plastic deformation tends to take place in these phases which constitute the grain boundary, and in many cases, it is difficult to produce sintered bodies which have high strength at high temperatures. For example, when MgO is added to Si.sub.3 N.sub.4, a vitreous phase of SiMgO.sub.3 results as a second phase, and fills the grain boundary to make the product highly dense. However, the product has the defect that since the vitreous phase softens at high temperatures, the mechanical strength of the sintered body abruptly begins to decrease at about 1000.degree. C. In the production of high-density sintered bodies by utilizing the plastication or liquefaction of additives, the strength of the product decreases markedly at high temperatures because of plastic deformation or liquid flow in the grain boundary. Among the conventional additives, those which would not cause a reduction in strength at high temperatures are other than the oxides which tend to become vitreous and the metallic elements which tend to become liquid. Solid powders of the carbides or borides, however, have poor self-sinterability, and cannot be expected to produce an effect of giving high-density sintered bodies.
It is an objective of this invention to provide a process for producing a heat-resistant ceramic sintered body having superior mechanical strength at room temperature and higher temperatures by using a new additive which densely fills the grain boundary and increases the sinterability of the ceramic substrate in the process of sintering, inhibits the growth of coarse particles of the ceramic, and which does not cause plastic deformation at high temperatures.
The strength of the ceramic sintered body in accordance with this invention is equal to, or higher than, those of conventional ceramic sintered bodies in spite of the fact that its density is lower than those of the conventional ceramic sintered bodies.
In the copending U.S. patent application Ser. No. 953,518 filed Oct. 23, 1978, the present inventors suggest a process for producing novel polycarbosilane, and disclose that this process can afford polycarbosilane partly containing siloxane bonds which has superior properties. They have now found that the above objective of the present invention is achieved by using as the additive the polycarbosilane partly containing siloxane bonds obtained by the process described in the aforesaid U.S. patent application.